Concerns over the safety of earlier lithium secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials as the negative electrode (anode). The carbonaceous material may comprise primarily graphite that is intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1. In order to minimize the loss in energy density due to this replacement, x in LixC6 must be maximized and the irreversible capacity loss Qir in the first charge of the battery must be minimized. The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal is generally believed to occur in a graphite intercalation compound represented by LixC6 (x=1), corresponding to a theoretical specific capacity of 372 mAh/g.
In addition to carbon- or graphite-based anode materials, other inorganic materials that have been evaluated for potential anode applications include metal oxides, metal nitrides, metal sulfides, and a range of metals, metal alloys, and intermetallic compounds that can accommodate lithium atoms/ions. In particular, lithium alloys having a composition formula of LiaA (A is a metal such as Al, and “a” satisfies 0<a<5) has been investigated as potential anode materials. This class of anode active materials has a higher theoretical capacity, e.g., Li4Si (maximum capacity=3,829 mAh/g), Li4.4Si (maximum capacity of Si=4,200 mAh/g), Li4.4Ge (maximum capacity of Ge=1,623 mAh/g), Li4.4Sn (maximum capacity of Sn=993 mAh/g), Li3Cd (maximum capacity of Cd=715 mAh/g), Li3Sb (maximum capacity of Sb=660 mAh/g), Li4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li3Bi (385 mAh/g).
An anode active material is normally used in a powder form, which is mixed with conductive additives and bonded by a binder resin. The binder also serves to bond the mixture to a current collector. Alternatively, an anode active material may be coated as a thin film onto a current collector. On repeated charge and discharge operations, the alloy particles tend to undergo pulverization and the current collector-supported thin films are prone to fragmentation due to expansion and contraction of the anode active material during the insertion and extraction of lithium ions. This pulverization or fragmentation results in loss of particle-to-particle contacts between the active material and the conductive additive or contacts between the anode material and its current collector. These adverse effects result in a significantly shortened charge-discharge cycle life.
To overcome the problems associated with such mechanical degradation, several approaches have been proposed, including (a) using nano-scaled particles of an anode active material, (b) composites composed of small electrochemically active particles supported by less active or non-active matrices or coatings, and (c) metal alloying. Examples of more promising active particles are Si, Sn, and SnO2. For instance, Umeno, et al. [“Novel Anode Material for Lithium-Ion Batteries: Carbon-coated Silicon Prepared by Thermal Vapor Decomposition,” Chemistry Letters, (2001) pp. 1186-1187] proposed an anode based on carbon-coated silicon prepared by thermal vapor decomposition. Although a specific capacity as high as 800-1,450 mAh/g was achieved, the capacity faded rapidly after 40 cycles. Hung [C. C. Hung, “Carbon Materials Metal/Metal Oxide Nanoparticle Composite and Battery Anode Composed of the Same, U.S. Pat. No. 7,094,499 (Aug. 22, 2006)] disclosed a method of forming a composite anode material. The steps include selecting a carbon material as a constituent part of the composite, chemically treating the selected carbon material to receive nanoparticles, incorporating nanoparticles into the chemically treated carbon material, and removing surface nanoparticles from an outside surface of the carbon material with incorporated nanoparticles. A material making up the nanoparticles alloys with lithium. The resulting carbon/nanoparticle composite anodes did not exhibit any significant increase in capacity, mostly lower than 400 mAh/g. In fact, most of prior art composite electrodes have deficiencies in some ways, e.g., in most cases, less than satisfactory reversible capacity, poor cycling stability, high irreversible capacity, ineffectiveness in reducing the internal stress or strain during the lithium ion insertion and extraction cycles, and some undesirable side effects.
The positive electrode (cathode) active material of a lithium-ion battery is typically selected from a broad array of lithium-containing or lithium-accommodating oxides, such as lithium manganese dioxide, lithium manganese composite oxide, lithium nickel oxide, lithium cobalt oxide, lithium nickel cobalt oxide, lithium vanadium oxide, and lithium iron phosphate. These prior art materials do not offer a high lithium insertion capacity and this capacity also tends to decay rapidly upon repeated charging and discharging. A practical specific capacity of a cathode material has been, at the most, up to 200 mAh/g of the cathode active material, which rapidly decays as the charge-discharge cycling operation proceeds. Since the cathode specific capacity is relatively low, there is a strong desire to make use of a cathode active material to its full capacity.
As a lithium-ion cell is charged and discharged, lithium is alternately stored in the cathode and in the anode, so that the total amount of cyclable charge corresponds to the amount of lithium shuttling back and forth between the two electrodes. When the cell is assembled, usually the cathode active material is made to store the amount of lithium available for the subsequent cyclic operation. This selection of cathode, instead of anode, to pre-store the needed lithium was mainly due to the notion that cathode active materials, such as lithium cobalt oxide, are relatively stable in ambient air (e.g., against oxidation) compared to lithiated graphite. However, the notion that this amount of lithium that determines the battery capacity is totally supplied from the cathode, limits the choice of cathode active materials because the active materials must contain removable lithium. Such a requirement prevents the use of other types of possibly higher-capacity cathode active materials; e.g. lithium-free chalcogen compounds, such as titanium disulfate or molybdenum disulfate, and transition metal fluoride, such as FeF3. Also, delithiated products corresponding to LiCoO2 and LiNiO2 formed during charging (e.g. LixCoO2 and LixNiO2 where 0.4<x<1.0) and overcharging (i.e. LixCoO2 and LixNiO2 where x<0.4) are not stable. In particular, these delithiated products tend to react with the electrolyte and generate heat, which raises safety concerns.
When the lithium-ion cell is assembled and filled with electrolyte, the anode and cathode active materials have a difference in potential of at most about 2 volts between each other. The difference in potential between the two electrodes, after the lithium-ion cell has been charged, is about 4 volts. When the lithium-ion cell is charged for the first time, lithium is extracted from the cathode and introduced into the anode. As a result, the anode potential is lowered significantly (toward the potential of metallic lithium), and the cathode potential is further increased (to become even more positive). These changes in potential may give rise to parasitic reactions on both electrodes, but more severely on the anode. For example, a decomposition product known as solid electrolyte interface (SEI) readily forms on the surfaces of carbon anodes, wherein the SEI layer comprises lithium and electrolyte components. These surface layers or covering layers are lithium-ion conductors which establish an ionic connection between the anode and the electrolyte and prevent the reactions from proceeding any further.
Formation of this SEI layer is therefore necessary for the stability of the half-cell system comprising the anode and the electrolyte. However, as the SEI layer is formed, a portion of the lithium introduced into the cells via the cathode is irreversibly bound and thus removed from cyclic operation, i.e. from the capacity available to the user. This means that, during the course of the first discharge, not as much lithium moves from the anode back to the cathode as had previously been released to the anode during the first charging operation. This phenomenon is called irreversible capacity and is known to consume about 10% to 30% of the capacity of a lithium ion cell.
A further drawback is that the formation of the SEI layer on the anode after the first charging operation may be incomplete and will continue to progress during the subsequent charging and discharge cycles. Even though this process becomes less pronounced with an increasing number of repeated charging and discharge cycles, it still causes continuous abstraction, from the system, of lithium which is no longer available for cyclic operation and thus for the capacity of the cell. Additionally, as indicated earlier, the formation of a solid-electrolyte interface layer consumes about 10% to 30% of the amount of lithium originally stored at the cathode, which is already low in capacity (typically <200 mAh/g). Clearly, it would be a significant advantage if the cells do not require the cathode to supply the required amount of lithium.
Takahashi, et al. [“Secondary Battery,” U.S. Pat. No. 4,980,250, Dec. 25, 1990], disclosed methods by means of which lithium is introduced into the anode active material in order to minimize the lithium consumption and thus the irreversible capacity loss. However, prelithiated carbon- or graphite-based anode active materials lead to electrodes which can be handled only under non-oxidizing and dry conditions, making practical production of lithium ion batteries difficult.
Meissner [E. Meissner, “Secondary Lithium-ion Cell with an Auxiliary Electrode,” U.S. Pat. No. 6,335,115 (Jan. 1, 2002)] disclosed a secondary lithium-ion cell which includes a lithium-intercalating, carbon-containing anode, a non-aqueous lithium ion-conducting electrolyte, and a lithium-intercalating cathode including a lithium-containing chalcogen compound of a transition metal, and a lithium-containing auxiliary electrode disposed in the cell to compensate for the irreversible capacity loss in the secondary lithium-ion cell. This auxiliary electrode is spatially separated from the electrolyte when the cell is positioned in a first orientation and contacts the electrolyte when the cell is oriented in a second position, for supplying additional lithium to the cell. Such an additional electrode makes the battery very complicated and difficult to make. Switching between two orientations is not a good strategy since it would complicate the handling of the battery and an average consumer would not pay attention to such a detail to ensure proper operation of such a battery.
The approach of using a sacrificial electrode, in addition to an anode and a cathode in a cell, was also proposed earlier by Johnson, et al. [“Rechargeable Lithium Ion Cell,” U.S. Pat. No. 5,601,951, (Feb. 11, 1997)]. Again, this additional electrode further complicates the manufacture and operation of a resulting battery. The assembling operation of a battery containing a highly reactive lithium metal or alloy electrode must be handled in an oxygen-free and moisture-free environment.
Gao, et al. [Y. Gao, et al., “Lithium Metal Dispersion in Secondary Battery Anode,” U.S. Pat. No. 6,706,447, Mar. 16, 2004 and U.S. Pat. No. 7,276,314 (Oct. 2, 2007)] disclosed a secondary battery containing an anode that is formed of a host material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material. The lithium metal is a finely divided lithium powder and preferably has a mean particle size of less than about 20 microns. The host material comprises one or more materials selected from the group consisting of carbonaceous materials (e.g., graphite), Si, Sn, tin oxides, composite tin alloys, transition metal oxides, lithium metal nitrides and lithium metal oxides. The method of preparing such an anode includes the steps of providing a host material, dispersing lithium metal in the host material, and then forming the host material and the lithium metal dispersed therein into an anode. The lithium metal and the host material is mixed together with a non-aqueous liquid to produce a slurry and then applied to a current collector and dried to form the anode. Alternatively, the anode can be formed by chemical means by immersing the host material in a suspension of lithium metal in a non-aqueous liquid, and then formed into an anode. The approach of Gao, et al. has the following drawbacks:                (1) The anode is composed of an anode active material (e.g., graphite or Sn particles) and a discrete lithium metal phase (fine Li metal powder particles) forming a mixture of two types of particles. This implies that the anode still contains highly active lithium particles that are sensitive to oxygen and moisture and must be handled under very stringent conditions.        (2) The amount of lithium metal present in the anode is prescribed to be no more than the maximum amount sufficient to intercalate in, alloy with, or be absorbed by the host material in the anode. For example, if the host material is carbon, the amount of lithium is no more than the amount needed to make LiC6. This implies that the amount of lithium needed for the SEI formation was not considered and, hence, the resulting battery will suffer a capacity loss of 10%-30% after the first cycle.        (3) It is of significance to note that although Gao, et al. mentioned in passing that the anode active material can be a mixture of carbon, Sn, Si, etc, the mixture still further contains lithium metal powder particles as an additional, discrete phase that is un-protected. The resulting multi-component, multi-phase mixture is still sensitive to the presence of oxygen and water contents in the air, thus significantly complicating the battery manufacturing operations.        (4) In a follow-on patent application, Gao, et al. [Y. Gao, et al. “Lithium metal dispersion in electrodes,” US Patent Application Pub. No. 2005/0130043 (Jun. 16, 2005)] suggested methods of lithiating an electrode prior to combining electrodes and other components to form a battery. In all cases, the electrode is composed of a mixture of discrete lithium metal particles or wire screen and powder particles of a host material, the latter being partially litiated. As shown in FIG. 1 of Gao, et al., the anode comprises discrete lithium metal particles and a host material. Both the discrete lithium metal particles and lithiated carbonaceous material (graphite) are unstable in an oxygen- or moisture-containing environment. Furthermore, Gao, et al. have not fairly suggested how other anode active materials than graphite can be prelithiated in a controlled manner (e.g., without inducing a lithium coating on the surface of active material particles). In fact, no example was given to illustrate if or how other important anode active materials can be successfully prelithiated prior to battery production. No battery testing or electrochemical performance evaluation data was given in any of Gao's patent or patent application to demonstrate the advantages of their electrodes.        
Therefore, there exists an urgent need for a secondary lithium ion battery that has one or more of the following features or advantages:    a) The battery does not contain a sacrificial electrode or an extra electrode in addition to an anode and cathode in a cell;    b) The battery comprises an anode that does not contain a discrete lithium metal phase or lithium metal powder particles dispersed in the anode;    c) The anode comprises at least two types of anode active material wherein at least one type of active material is prelithiated (e.g., Si and Sn) and at least one type of active material is not prelithiated (e.g., carbonaceous material, such as graphite, hard carbon, soft carbon, surface-modified graphite, chemically modified graphite, or meso-carbon micro-beads, MCMBs).    d) The battery contains an anode that comprises at least a non-carbon active material possessing an ultra-high lithium absorbing capacity (e.g., Si that exhibits a specific capacity up to 3,500 or even 4,200 mAh/g);    e) The battery comprises an anode that contains an excess amount of lithium (disposed inside a non-carbon anode active material, not on its surface) to compensate for the formation of SEI layers, in addition to providing enough lithium to intercalate into (or form a compound with) a cathode active material.    f) The battery features a long and stable cycle life due to an anode that comprises fine, prelithiated active particles capable of maintaining their integrity and their contact with the conductive additive and the current collector.
Therefore, in summary, a need exists for an anode active material that has a high specific capacity, a minimal irreversible capacity (or a low decay rate), and a long cycle life. In order to accomplish these goals, we have worked diligently and intensively on the development of new electrode materials. These research and development efforts lead to the present patent application.